This application claims priority from Korean Patent Application No. 2002-0044442 filed on Jul. 27, 2002.
The present invention pertains, in general, to fabrication methods of UBM (under bump metallurgy) for flip chip interconnections of semiconductor devices, and in particular, to a method of fabricating the desired UBM by controlling current density during electroplating and without using an etching process.
Generally, as interconnection techniques between a semiconductor integrated circuit chip and a substrate, there have been employed the processes of flip chip, wire bonding and TAB (tape automated bonding). Of them, the flip chip process is advantageous in terms of high-speed and high density connections due to higher connection density and shorter connection distance, compared to the others. As for the flip chip interconnections, a UBM layer is bonded to a pad (aluminum or copper) of the chip, after which UBM is further bonded to a conductive solder bump. The bump-formed chip functions to bond the solder bump and the metal layer on the substrate, whereby electric signal transmission between the chip and the substrate can be accomplished. As such, a mechanical bonding is practicable. UBM provides good adhesion between the solder and the pad of the chip eliminates much of the mutual diffusion between the solder and the substrate.
Such a flip chip connection technique was devised for the first time by IBM in the 1960s. The fabrication method of those days comprised forming UBM with chromiun/chromium-copper alloy/copper layers on the chip pad by evaporation and then forming Sn-95% Pb solder bumps thereon by evaporation. In such a structure, the chromium layer is provided to be bonded to the pad, and the chromium-copper alloy layer prevents mutual diffusion of the solder and the pad. In addition, the copper layer is introduced to increase bondability and wettability with the solder. The above method, excellent in reliability, has been used for a considerable period. However, in typical electronic parts, the solder of 63% Sn-37% Pb having higher Sn content is mainly used, compared to Sn-95% Pb solder. In recent years, the process trended toward using environmentally safe materials, such as a Pb-free solder. Many problems are caused in the flip chip process including the finding that the UBM structure cannot effectively prevent the diffusion between the solder and the pad upon use with 63-37% Pb process solder and Pb-free solder, and in particular, Pb-free solder. Many researchers are working on UBM suitable for use with Pb-free solder, using sputtering, electroplating and electroless plating methods.
Most Pb-free solders developed until now have a large amount of tin. The Pb-free solder materials, suitable for use in the flip chip interconnections, are exemplified by Sn-3.5% Ag, Sn-0.7% Cu, and Sn-3.8% Ag-0.7% Cu. These materials contain 95% or more of Sn. Since the tin element rapidly reacts with copper, tin in the solder reacts with copper of UBM by heat generated in the course of reflow of the flip chip or use of the chip. Thus, an intermetallic compound is formed at the interface of UBM and the solder, and the copper is self-extinguished. If the intermetallic compounds are excessively formed or the copper layer in the UBM is completely self-extinguished, bonding strength between the solder and the pad is drastically decreased. Hence, UBM for use with Pb-free solder having high Sn content requires a novel diffusion barrier. In this regard, nickel is used. Nickel is slower in reaction rate with tin than copper. Until now, there have been proposed various processes for the formation of the diffusion barrier made of nickel or nickel alloy through sputtering, electroplating and electroless plating methods. However, the UBM structure including the nickel layer suffers from the problems related with poor solderability of nickel and residual stress in the nickel layer. In this regard, since nickel is low in wettability and bondability with the solder, it should be coated with gold or copper to obtain sufficient wettablity. Also, different from the copper layer, the nickel layer has high residual stress due to intrinsic properties of nickel and processing characteristics of the plating process. In the case where nickel is applied to UBM of silicone chip, the silicone chip may be cracked due to residual stress in UBM containing the stressed nickel. So, the residual stress in the UBM layer or thickness thereof should be decreased to prevent such cracks. However, if the thickness of the UBM is excessively reduced, the UBM layer may be completely self-extinguished upon the interfacial reaction with the solder. Therefore, the thickness of the UBM layer should be maintained at a predetermined level.
UBM for use with Pb-free solder in flip chip interconnections should meet requirements such as good wettablility and bondability with the solder, slow reaction with the solder to prevent the diffusion between the solder and the chip pad, and low residual stress not to cause cracks of the chip. With the aim of exhibiting the above requirements, the UBM structure comprises the multilayered structure as in the following Table 1.
The lower layer made of aluminum, chromium or titanium is responsible for maintenance of bondability with the pad of the chip, and the intermediate layer 1 formed of aluminum or copper decreases the residual stress in UBM. The intermediate layer 2 formed of nickel or nickel alloys prevents diffusion, and the upper layer made of gold or copper provides wettability with the solder.
As representative UBM structures for flip chips, there are chromium/copper-chromium alloy/copper, titanium-tungsten alloy/copper/electrolytic copper, aluminum/nickel-vanadium alloy/copper, electroless nickel-phosphorus alloy/gold. Of them, chromium/copper-chromium alloy/copper structure, which was developed by IBM, is known not to be used with the Pb-free solder. In the structure of titanium-tungsten alloy/copper/electrolytic copper developed for use with Sn-37% Pb solder, the higher the Sn content as in the Pb-free solder, the thicker the electrolytic copper layer. Thus, the above structure has high residual stress and cannot be used. In the case of aluminum/nickel-vanadium alloy/copper structure developed to use with Pb-free solder by Delco, the nickel-vanadium layer is known to be slow in reaction with the Pb-free solder, but the used sputtering method requires an etching process, and suffers from higher process cost, compared to electroplating method. The structure of electroless nickel-phosphorus alloy/gold is advantageous in low fabrication cost but has the disadvantage of brittleness of the electroless nickel layer upon reaction with the Pb-free solder.
Therefore, it is an object of the present invention to solve the problems in the prior art and to provide a method of fabricating multi-layered UBM having a copper/nickel-copper alloy/copper structure via electroplating.
In accordance with one embodiment, the present invention provides a method of fabricating UBM for flip chip interconnections of a semiconductor device. The method comprises dipping a patterned wafer into a plating solution containing a material source of nickel and copper ions. A copper layer is formed at a predetermined current density for connection between a chip pad and a solder bump. The copper layer is of sufficient thickness to provide residual stress-buffering on the wafer. A nickel-copper alloy layer is formed, at an increased current density, and having sufficient thickness for prevention of mutual diffusion between the solder and the pad on the copper layer. Another copper layer is optionally formed on the nickel-copper alloy layer at a decreased current density for improvement of wettability with the solder. The ratio of the current density required to form the copper layer and the nickel-copper layer preferably ranges from 1:2.5 to 1:10.
The material source of supplying the nickel ion include NiSO4, NiCl2, Ni(SO3NH2)2, and combinations thereof. The material source of supplying the copper ion include CUSO4, CuCN, Cu2P2O7 and combinations thereof.